This invention relates to communications switching networks and more particularly to providing portability to station numbers within the network.
Within the public telecommunications switching network, there is an increasing need and desire to provide station number portability such that a subscriber may retain a given station number regardless of where the subscriber and station physically reside and which switch provides service.
Presently, the public switching telephone network (PSTN) relies on a relationship between the station number (Directory Number--DN) and the switch node that it is served by. Basically, the public numbering plan is organized as follows:
All DN's are of the form NPA-NXX-XXXX. PA0 Each NPA is a three digit code representing a geographic area of the country. This is commonly referred to as the Area Code. PA0 Each NXX is a three digit code loosely representing the Office Code of the serving switching node within each NPA (more specifically defined, an office code is formed by NPA and NXX). A node may serve more than one Office Code (i.e. NXX). However, typically only one switch node may serve a given NXX, hence, the uniqueness of the relationship of NXX's to switch nodes. PA0 The XXXX is a four digit station number which provides a pointer or identification to the station's appearance on a given switch node. Combined with the associated NXX code, it gives a unique identification of a number within a given serving area (NPA), hence a Directory Number (DN) is formed.
When a customer dials a number (either 7 or 10 digits excluding prefixes), the switch node of the originating station is able to translate the dialled digits into a location for the called number, be it a location on the same switch or a route out of the switch to another switch node elsewhere in the switch network. Typically, the translations are setup to take advantage of the fact that office codes (NXX's ) uniquely identify the switch node that the call is intended for. Thus, once the office code is determined, action is taken to route the call--either to route within the switch node (based on analysis of the station digits) or to choose an outgoing route to another switch node. It is the relationship of office codes (NXX's ) to switch nodes that Local Number Portability (LNP) affects. To determine the correct switch node in the entire public network requires analysis of the full 10 digits in the dialled number.
Various proposals for LNP using an Advanced Intelligent Network (AIN) based solution have been suggested in "Generic Switching and Signalling Requirements for Number Portability" (FSD30-12-0001) dated Nov. 20, 1995 by the Illinois Number Portability Workshop. These AIN based LNP proposals rely on a query to an off-switch (i.e., "off-board") database which is used to track movement of numbers. Thus with AIN LNP, during each call event (i.e. origination), the originating switch must make a determination as to whether the called number is indeed on the switch or elsewhere in the switch network. If it is not on the originating switch, then the originating switch node (or some switch node) must "query" the "off-board" database to find out where the number is so that the call can be routed on the appropriate trunk group into the communication network. It does this on an on-going basis for any calls to "ported" numbers not located on the switch, even for numbers that are on the same switch they were on at the last query; there is no way to tell that a number has moved. It is this "continual" querying that adds substantial processing and costs to the network. In addition, because local area calling (or community of interest--COI calling) forms the majority of calls of the typical communication network, the added cost is significant for the operating company to provide local service for which the operating company typically receives a fixed monthly charge independent of usage.
Another proposal for Number Portability is made in U.S. Pat. No. 4,754,479, Bicknell et al, issued Jun. 28, 1988. According to this proposal, every switch in a portability cluster contains a common database which lists the station numbers in the cluster and specifies the particular switch connected to these station numbers. When a number is ported the common database is altered to indicate the new switch to which the ported number is now connected.
Bicknell et al suffers from the disadvantage that it requires a higher number of process steps adding to the complexity and response time of the system. In particular, it is necessary to establish by scanning a portability cluster table whether the switch to which the calling station is connected is part of the portability cluster. At a later stage it is necessary to translate the called station DN into an associated switch identity in the common database to indicate which switch the called station is physically connected to. Thus, even if a call is going from a first station physically connected to a switch to a second station physically connected to the same switch, both of the steps must be carried out. In addition, calls to unassigned numbers have to be routed to designated switches even though the number is not used.